This invention relates to the field of target grid assemblies for use with accelerators for the production of radioisotopes.
Positron Emission Tomography (PET) is a powerful tool for diagnosing and treatment planning of many diseases wherein radioisotopes are injected into a patient to diagnose and assess the disease. Accelerators are used to produce the radioisotopes used in PET. Generally, an accelerator produces radioisotopes by accelerating a particle beam and bombarding a target material, housed in a target system, with the particle beam.
Several factors must be considered when developing a target system for the production of radioisotopes. In the case of gas or liquid targets, the target material must be maintained at an elevated pressure during bombardment to compensate for the effects of density reduction of the target material due to heating/expansion/phase change (boiling). Further, it is desirable to operate at higher beam currents to increase production of the radioisotopes. Because of the amount of heat generated during bombardment, cooling the target material and other components of the target system is of significant importance.
A typical target system includes a target window sealed against a target cavity with the target material retained in the target cavity. The target window is necessarily thin to minimize energy loss to heat generated in the target window. The design constraint of making the target window as thin as possible is balanced by the need to maintain the target material at an elevated pressure during bombardment, as stated above. Because the target window must support higher pressures, a lower limit is placed on the thickness of the target window to maintain a given pressure on the target side. Moreover, the desirability of operating with higher beam currents and increased beam size also conflicts with the minimization of the thickness of the target window. More specifically, one simple way to accomplish increased beam production is to increase the physical size of the beam which necessitates making the target window larger, but the maximum pressure sustainable by a window decreases linearly with window area. Conventional target system designs at present have maximized the available technology with regard to window thickness, selection of window materials and overall target performance such that operating at higher pressures and increased beam sizes is not possible with these conventional target systems.
Target systems developed more recently include the use of a thick high transparency grid to support the target window. The grid breaks the window into several smaller windows of significantly smaller area, thereby increasing the maximum pressure the target window material can withstand such that, with windows of the same material and thickness as currently used, the bean size can increase, as well as the target operating pressure. However, there are additional design constraints introduced by the grid. Specifically, the grid must be thick enough in the beam propagation direction to mechanically support the target window. In many cases, the thickness of the grid is thicker than the distance the beam travels in typical grid candidate materials. Therefore, any beam that is intercepted by the grid does not reach the target material to produce the desired radioisotope. For this reason, the grid transparency must be maximized, primarily by reducing the thickness of the grid septa (webs or supports). The best strength/transparency geometry is hexagonal grid cells, a configuration which is well known in the art and has been applied to numerous target system designs and illustrated in FIGS. 1a and 1b. A drawback to the hexagonal grid is the difficulty involved with manufacturing the grid.
Utilizing a grid to support the target window introduces additional complications with respect to cooling. Regardless of the target design, heat is generated in the target material during the bombardment process. This heat is conducted to and through the target window. When utilizing a grid, the impinging beam heats the grid which in turn conducts heat to the target window and the target material. It has been shown that, if the grid is cooled at its edges by recirculating water or other coolants, the heat from the intercepted beam onto the grid, the heat deposited in the target window and some of the heat from the target material can be removed by the grid. However, only modest amounts of heat is removed from the target material with this method of cooling. Moreover, even with a conventional, non-gridded target window wherein the window is helium cooled, the heat removed from the target material is modest compared with the heat generated in the target material. To incorporate the helium cooling of the conventional target design into the hexagonal grid support design, the grid would have to be mechanically very complex to produce jets that impinge on each sub-window. Moreover, when keeping the septa small (on the order of 0.010xe2x80x3 thick), it would be impossible to achieve impinging helium jets for each sub-window.
It has been shown that the highest temperature and therefore, the most density-reduction prone region of the target material is very near the target window. Therefore, to increase yields and length of service intervals and to improve reliability, significant effort should be dedicated to better cooling of the target window.
Therefore, it is an object of this invention to provide a target grid assembly which defines a configuration such that helium cooling to cool the target window and target material can be employed.
It is another object of the present invention to provide a target grid assembly which can remove more heat from the target material than the target grid assemblies of the prior art.
Further, it is an object of the present invention to provide a target grid assembly which can withstand higher pressures and higher beam currents than the non-gridded assemblies of the prior art.
Moreover, it is another object of the present invention to provide a target grid assembly wherein the thickness of the grid septa or supports is minimized.
It is yet another object of the present invention to provide a target grid assembly which includes a grid which defines a simpler design and is easier to manufacture than the hexagonal grid of the prior art, while being capable of providing support to the target window similar to that of the hexagonal grid.
Other objects and advantages will be accomplished by the present invention which serves to provide a target grid assembly for employment in a target assembly used to produce radioisotopes by bombarding a target material contained in the target assembly with a particle beam. The target assembly includes the target grid assembly, a target window and a target body enclosed in a target housing. The target body defines a target reservoir for retaining the target material and the target window is positioned against the target body to cover and seal the target reservoir.
The target grid assembly includes at least a target grid which serves to support the target window. The target grid defines a target grid portion defining a plurality of target grid supports which are configured to form a plural of target grid oblong openings. In the preferred embodiment, the target grid assembly further includes a vacuum window and the target grid further defines a helium input and a helium output. The vacuum window is positioned against the upstream side of the target grid portion and the target window and target body are positioned on the downstream side of the target grid. A helium space is defined by the plurality of target grid oblong openings between the target window and the vacuum window and is configured such that helium is injectable into the helium space via the helium input and extractable from the helium space via the helium output to form a helium cooling regime.